In the field of civil engineering, every reinforced concrete structure we encounter—ranging from high-rise buildings and bridges to residential houses—begins with a crucial yet often overlooked phase: preliminary design.
This phase marks the beginning of a structural planning process that is efficient, safe, and economical. This article discusses what preliminary design is, why it is important, and how to perform it with reference to ACI 318-14.
What is Preliminary Design?
Preliminary design is the initial stage in the planning of a reinforced concrete structure, where approximate dimensions of major structural elements—such as slabs, beams, columns, and foundations—are determined based on assumed loads, structural system, and material properties.
The purpose is not to produce a final design, but to establish a near-realistic framework that facilitates further detailed structural analysis.
Why is Preliminary Design Important?
- Prevents overdesign or underdesign of structural elements.
- Ensures that the structure can support the expected design loads.
- Provides a foundation for advanced structural modeling and analysis (e.g., using SAP2000, ETABS, etc.).
- Saves time and cost during the design development stage.
Preliminary Design Guidelines
Design codes for preliminary design vary depending on governmental regulations. In Indonesia, preliminary design follows SNI 2847:2019 (Requirements for structural concrete for buildings and commentary), which adopts ACI 318-14 (Building Code Requirements for Structural Concrete) and ACI 318R-14 (Commentary).
Preliminary design formulas for beams vary depending on the moment-resisting frame system being used:
- Special Moment Resisting Frame (SMRF)
- Intermediate Moment Resisting Frame (IMRF)
- Ordinary Moment Resisting Frame (OMRF)
See also: What is a Moment Resisting Frame System?
Prestressed and Non-Prestressed Concrete
Before discussing the formulas used in preliminary design, it is important to distinguish two commonly used terms, especially for students new to structural design. The codes refer to two main systems of concrete:
- Non-Prestressed Concrete, which gains strength from conventionally reinforced steel bars embedded in the concrete.
- Prestressed Concrete, which gains strength from tensioned steel tendons.
Now, let us discuss the preliminary design equations for beams on the next page.